The Mayor of London’s office has strenuously defended the city’s record on air pollution, insisting that it is far from the worst polluted city, despite data showing that nitrogen dioxide (NO2) levels have been higher than anywhere else yet recorded.

Boris Johnson’s top air quality officials told the Guardian that it was “ludicrous” to put London in the same category as capitals such as Mexico City, Delhi and Beijing, and other cities.

“It’s not even comparing apples with pears, it is comparing apples with cucumbers,” said Matthew Pencharz, the mayor’s adviser on environment and energy.

Pencharz claimed other cities frequently site their air quality monitoring stations far from the sources of the worst pollution, for instance in parks, quiet roads or suburbs, but that London’s monitoring stations are “more reliable” because they are sited near busy roads.

Levels of NO2 on Oxford Street in 2013 were at an annual mean of 135 μg m-3, according to samples taken by the London Air Quality Network.

Other data analysed by the European Environment Agency also showed London topping Europe’s cities for air pollution, though at lower levels than the Oxford Street samples, which were not submitted for the agency’s consideration.

Under the EEA’s analysis, Marylebone Road, a busy London road where pollution levels frequently exceed the EU’s legal maximum, was found to be the worst in Europe in 2012, with 94 μg m-3, compared to a maximum of 40 μg m-3 which is considered by the EU to be the upper safe limit for human health. The next worst European cities were Stuttgart, with 91 μg m-3 at one site and 90 μg m-3 at another, and Paris with 90 μg m-3 at one site.

NO2 data

But Pencharz said that the data cited was not representative, because other cities have fewer monitoring stations and submit less data. London had more monitoring stations than Paris or Rome, for instance, with 157 sites compared to 32 in Paris, 13 in Rome, 17 in Berlin and 19 in Amsterdam, in 2011. He also said other cities frequently sited their monitoring stations away from some of their main polluting sites, and therefore were not as reliable as the UK.

The mayor’s office said: “More monitoring stations means we can do the responsible thing and place our monitoring stations directly in the most polluted roads. Some cities place their monitoring stations in vast green parks or traffic-free courtyards, hence the lower figures they report. That’s why Oxford Street records comparatively high levels of pollution than for many other cities – but to say it is the worst of any place on Earth is misleading.”

Martin Adams of the European Environment Agency confirmed that member states could pick the data they wanted to submit on air quality, provided that it met certain criteria and a minimum number of sampling stations were included. “That is the flexibility that member states have,” he told the Guardian. “That is the responsibility of member states – to choose the monitoring stations [from which they submit data].”

NO2 can contribute to breathing difficulties, and is a particular problem for young children, older people and susceptible adults, such as those with existing respiratory conditions. Diesel-fuelled vehicles are a major source of the pollutant, and London is facing hefty fines from the European Union for flouting NO2 limits.

Johnson unveiled plans on Tuesday to double the current congestion charge for drivers of diesel-powered vehicles. He said that diesel was worse for air pollution than alternatives including petrol, electric vehicles and other transport. The new plans – which must be put out to consultation before they can be adopted – would not come into force until 2020.